The present invention relates to an apparatus for monitoring defects in prints printed, i.e. printed material, by, for example, an rotary offset press.
Conventional apparatus for monitoring defects in prints are disclosed, for example, in Japanese Patent Laid-Open Publication Nos. 60-58535 and 56-98638. In such apparatus, a contamination or the like formed on a print surface is observed or monitored with a detection sensor which extends perpendicularly to the direction in which the print surface is moved. As the print surface is moved, it is scanned with the detection sensor in synchronization with its movement to observe or monitor the whole area of the print surface with respect to linear sections thereof.
If a defect is discriminated, the position at which the defect has occurred, the cause of the defect and other kinds of information are displayed on a screen of a display unit such as a CRT, and a marking circuit is operated according to the content of the defect to mark the corresponding print portion of a print web by means of spraying (disclosed in, for example, Japanese Patent Laid-Open Publication No. 60-155465).
These conventional apparatus detect only the position of contaminations and cannot discriminate the contents of contaminations. Defects in the print surface are not limited to those occurring at arbitrary times and at arbitrary positions, e.g., a spatter of ink, and drops of water or oil. There are other defects such as density unevenness occurring in the direction of the flow of the print web by a cause relating to the adjustments of an ink control unit of the printing machine, and a streak-like defect occurring in the direction of the flow by a blanket failure or the like. Density unevenness of a streak-like defect is continuous unlike the transitory defects, i.e., a spatter of ink and drops of water or oil and must be removed by adjusting the printing machine.
The conventional print monitoring apparatuses therefore entail the following problems.
First, since only the defect position is indicated, it is difficult to discriminate whether the defects are single-occurrence phenomena or continuous phenomena.
Second, in the case of making a print, it is necessary to exatract a defective sample each time a defect occurs. It is therefore difficult to ascertain the cause, so that the finding of the print hindrance cause is retarded, resulting in an increase in printing cost.